Waterfall inline with RTA

[ashjmckenzie]

Suggesting a feature that mimics the onscreen waterfall of the tinySA, so we have spectral history as well as the Realtime, the current implementation of the waterfull is nice, but a flat view inline with the exsiting freq scale is so much faster for diagnostics

[g4ixt]

I don't know how to create a waterfall plot, although there are some examples of code in GitHub that I might be able to clone. I may have a look at this eventually. The existing 'time spectrum' (3D) is poor and I would like to improve that some time if I can, but it's hard to understand surface plots and there are no nice features like axis scales for them in pyqtgraph.